1. Technical Field
The present invention relates to wireless communications, and more particularly, to a method of transmitting control information for antenna selection in a multiple antenna system.
2. Related Art
To maximize performance and communication capability of a wireless communication system, a multiple input multiple output (MIMO) system has drawn attention in recent years. Being evolved from the conventional technique in which a single transmit (Tx) antenna and a single receive (Rx) antenna are used, a MIMO technique uses multiple Tx antennas and multiple Rx antennas to improve transfer efficiency of data to be transmitted or received. The MIMO system is also referred to as a multiple antenna system. In the MIMO technique, instead of receiving one whole message through a single antenna path, data segments are received through a plurality of antennas and are then collected as one piece of data. As a result, a data transfer rate can be improved in a specific range, or a system range can be increased with respect to a specific data transfer rate.
The MIMO technique includes transmit diversity, spatial multiplexing, and beamforming. The transmit diversity is a technique in which the multiple Tx antennas transmit the same data so that transmission reliability increases. The spatial multiplexing is a technique in which the multiple Tx antennas simultaneously transmit different data so that data can be transmitted at a high speed without increasing a system bandwidth. The beamforming is used to add a weight to multiple antennas according to a channel condition so as to increase a signal to interference plus noise ratio (SINR) of a signal. In this case, the weight can be expressed by a weight vector or a weight matrix, which is respectively referred to as a precoding vector or a precoding matrix.
The spatial multiplexing is classified into single-user spatial multiplexing and multi-user spatial multiplexing. The single-user spatial multiplexing is also referred to as a single user MIMO (SU-MIMO). The multi-user spatial multiplexing is also referred to as a spatial division multiple access (SDMA) or a multi user MIMO (MU-MIMO). A capacity of a MIMO channel increases in proportion to the number of antennas. The MIMO channel can be decomposed into independent channels. If the number of Tx antennas is Nt, and the number of Rx antennas is Nr, then the number of independent channels is Ni where Ni≦min{Nt, Nr}. Each independent channel can be referred to as a spatial layer. A rank represents the number of non-zero eigen-values of the MIMO channel matrix and can be defined as the number of spatial streams that can be multiplexed.
Control information for data transmission is transmitted to a user equipment (UE) through a downlink control channel. Downlink control information includes various types of information required for data transmission and reception. The UE may transmit data by receiving the control information through the downlink control channel. A UE having multiple antennas may transmit data through all or some antennas. Tx antenna selection of the UE may be indicated using the control information transmitted through the downlink control channel, or may be determined according to a predetermined rule. For example, a base station (BS) may provide a UE having two Tx antennas with control information regarding selection of a first antenna or a second antenna. Alternatively, the Tx antenna may be selected by the UE on a slot or subframe basis.
At present, a UE having 4 Tx antennas is considered in the multiple antenna system. The UE may selectively transmit and receive data by using some of the 4 Tx antennas. To support the 4 Tx antennas, the UE may employ 4 power amplifiers. However, this may be ineffective when data is transmitted and received using some of the Tx antennas, and a cost of the UE may increase as a result. Among the 4 Tx antennas, some of them having high Tx/Rx efficiency may be selected for the effective use.
Accordingly, there is a need for a method of selecting some antennas to be used in data transmission and reception among a plurality of antennas employed in a UE, and a signaling method related thereto.